In this study, a novel magnetic sludge biochar (MSBC) from sewage sludge was created by the assembly of strontium hexaferrite (SrFe12O19) onto the surface of sewage sludge biochar (SBC) under high-temperature and oxygen-free conditions. The characterization of MSBC was achieved by Fourier transform infrared spectroscopy, X-ray diffraction and vibrating sample magnetometry, and the adsorption properties of the MSBC towards malachite green (MG) from aqueous solution were systematically investigated. The influence of variables (different mass ratio of SBC and SrFe12O19, initial MG concentration, absorbent dosage, pH and contact time) was also studied in detail. The optimal adsorption amount of MG (388.65 mg MG/g) was obtained with 500 mg MG/L, 2.0 g MSBC/L for 40 min under pH of 7.0, with different mass ratios of SBC and SrFe12O19 (1:4, 1:2, 3:4 and 1:1), when the mass ratio of SBC and SrFe12O19 was 3:4 at room temperature, and the Langmuir model was more suitable than the Freundlich model for equilibrium data. Meanwhile, the kinetic models showed that the overall adsorption process was better described by a pseudo-second-order kinetic model. The results indicated that the MSBC was a novel, efficient, magnetically separable adsorbent for the removal of the dye from wastewater.